The attachment shows the appearance of the Ekco T221 which was brought to the shop today for service by forum member Derren.I have fond memories of this model as a similar set was bought by a relative who lived in Portslade, near Hove. The set was purchased in early 1955, I remember that the set was tuned to the temporary transmitter at Truleigh Hill. The channel number was B2.The Ekco employs a 14" CRT, the Mazda CRM141 which has a round screen and unlike the Mullard 14" CRT which has a rectangular faceplate.

Just a bit of background, the set was bought at auction and is in much above average cosmetic condition, even has the original dedicated mains connector. Restored by myself/partner a couple of years ago. Has worked very well indeed superb tube bright picture lots of contrast, excellent linearity and frame/line hold. However on last power up there was no line output, I decided to hand it over to Till rather than start taking it apart again, especially if the knobs need to come off again as that was a difficult part, it looked like the inner part on the volume side had possible been cracked/repaired by someone once before so we had to be extremely careful. Hopefully they won't have to come off again Only disadvantage of being a relative newcomer and not in the trade is some components are hard to find and sometimes have to be combined to get the right value, but we must have done something right for it to have worked so well for so long! If I'm not mistaken this is the set where someone had previously replaced all of the dropper resistors which seemed rather unusual.

First observations confirm that the line output transformer housing is in good condition. The 20P4 is exhibiting a blue glow on certain parts of the glass bulb, it's the kind of glow that indicates a particularly good vacuum rather than a gassy valve. No spark is present at the anode of the U25. I'll unsolder this valve and see what happens.

5:20pm:As I expected there is no drive to the grid of the 20P4 line output valve. The line multi-vibrator consists of one section of a 20L1 double triode and the screen grid of the 20P4, that forms the oscillator start up circuit. When fully functioning oscillator feedback takes the form of a 15pF pulse capacitor connected between the grid of the triode and a tapping on the line output transformer.

7:42pm: The 0.01microfarad coupling capacitor between the 20L1 triode and the grid of the 20P4 was found to be leaky, in fact very leaky! So it follows I thought I'd be on the money after replacing it and this action would effect a cure, 'fraid not, it's known that the 100Kohm triode anode load resistor is high in value @ 150Kohms. Nevertheless, the anode voltage of the 20L1 triode falls by only 10volts after warming up, that valve needs replacing. As soon as one is found I'll report back my findings after fitting the replacement.

Sounds like good progress. I am fairly sure I have a 20L1 and one of my attempts at fixing it was to substitute that valve, but clearly there was more to it than that. Was the 0.01 cap a one that had been replaced?

Hi Derren, The capacitor was the original TCC "metalmite" part. I found a 20L1 in the garden workshop so it will be fitted this evening when I return to the shop. Couldn't find one in there.Hopefully, the 20P4 has survived it's over current ordeal.These Mazda 20 series valves are becoming scarce.

The failed line oscillator was caused by a faulty 20L1. I've replaced it and now there is drive to the grid of the 20P4. However, only a tiny spark is present at the U25 EHT rectifier. Remove the top cap from the U301 boost diode and the EHT comes up. it's most likely to be a short circuit boost capacitor. According to the Wireless and Electrical Trader TV service supplement 1218/T85 the boost capacitor is designated as C48, 0.5microfarads. No such circuit reference to the boost capacitor in the 1955/56 Radio and Television servicing book.

Till Eulenspiegel wrote:According to the Wireless and Electrical Trader TV service supplement 1218/T85 the boost capacitor is designated as C48, 0.5microfarads. No such circuit reference to the boost capacitor in the 1955/56 Radio and Television servicing book.

I think the Trader not only redrew all circuits in their own style but adopted their own component references.

This can cause problems here when the member who has a problem is using a circuit from a different source to a member who is trying to help!

It is always useful to include a relevant snippet of the circuit in posts to avoid such confusion.

Terrykc wrote:I think the Trader not only redrew all circuits in their own style but adopted their own component references.

This can cause problems here when the member who has a problem is using a circuit from a different source to a member who is trying to help!

It is always useful to include a relevant snippet of the circuit in posts to avoid such confusion.

They did - often including the caveat that their component designations would be different to that of the manufacturer, so the source of the designation should be quoted when contacting the manufacturer's service department.

I know someone who can only work from Trader sheets, finding others 'confusing'.

Normally these are the easiest sets to get going, however this one is proving to be the exception. After replacing the boost capacitor other strange things started to happen. The line oscillator would not start up unless it was shocked into doing so, like switching the set off and on a few times. This action would usually get the line oscillator started. Another problem is the line drive waveform was not continuous taking the form of bursts. Squegging in fact. The EHT spark is also very weak. Well I've never replaced a line OP transformer in these sets but what about the scan coils? These have been disconnected and now there is a flame like spark present at the anode of the U25 EHT rectifier. Also the squegging has stopped and now there is a continuous line drive waveform present at the grid of the 20P4.

In a previous post I suggested that the scan coils might be faulty.The scan coil leads from the frame output transformer have been disconnected. Sure enough it's safe to say that insulation between the line and frame coils has broken down. Now there is a healthy spark at the U25 anode terminal. That valve is not connected up yet because I've got doubts about the visconol EHT capacitor.The attachment shows the circuit diagram of the timebases as taken from the 1955/56 Radio and Television servicing book. Note how the presentation differs from the Trader service information for the same set.

I generally used the manufacturers manuals, usually had more information in them unless a Trader sheet was all that was available.I think some of it is what you get used to, I know looking at manuals for USA TV's takes me sometime to get my head around them. Circuits very similar to UK but drawn completely different. Some part of the set had designs not used by UK sets but even allowing for that I find them difficult at first.

Till Eulenspiegel wrote:The attachment shows the circuit diagram of the timebases as taken from the 1955/56 Radio and Television servicing book. Note how the presentation differs from the Trader service information for the same set.

There's a mistake there, surely?

The heater section of the mains dropper is across the HT rectifier, with the heater chain connected to the central point!

In addition, there are two input connections to the dropper, effectively shorting most of it out and leaving MR2 in parallel with a 66Ω resistor to feed the HT line!

Removing the link from the top 66Ω resistor to the cathode of MR2 would restore the DC HT line but which is the correct connection point for the mains input?

The junction of the 60Ω and 20Ω resistors seem to make sense but where does the input of MR2 go? There is no connection to the lower chain of resistors, which would seem the logical choice.

In any event, the voltages for the dropper connections aren't shown, either!

Can you post the relevant section of the Trader sheet for comparison? This is doing my head in!

Did you notice Till that all of the dropper resistors had been replaced in this set? That was the case before I even got to it. Whatever is happening now the set worked very well indeed for about 18 months being used quite often.

With regard to the 1955/56 R & T servicing book's circuit diagrams of the T221 it looks as the mains voltage tappings are set for 200 volts.A bit unusual that.Getting back to the set on the bench, the scanning coils are definitely faulty. A replacement set of coils will have to be found.

Till Eulenspiegel wrote:With regard to the 1955/56 R & T servicing book's circuit diagrams of the T221 it looks as the mains voltage tappings are set for 200 volts.

The mains goes straight to MR2, which supports your theory but the heaters are shown for 220V, surely?

Another little quirk which made it even harder to unravel what was going on!

That string of resistors in the HT feed is very unusual as no two resistors have the same value! However, there is obviously a 10V tap on the heater chain and you could get 40Ω + 40Ω + 20Ω out of it to match. Could the odd arrangement be to produce a different set of values for DC mains operation, I wonder?

Hi Mike, That's good news, it's the same part just wired differently. For the time being the set has been taken off the bench. The cabinet is immaculate and don't want any harm to come to it. You might have read about my reminiscences on the other forum about a similar set sixty years ago. That dates me!

Collected the scan coils from Mike's on Thursday evening. Instead of removing the scan coils from a TMB272 the scan coil assembly was removed from the chassis of an Ekco T327. The coils are essentially the same as those in the T221 except the line coils are connected directly in series instead of the arrangement in the T221 in which the linearity and width coils are connected between the pair of line deflector coils.All that is required to use the replacement scan coils in the T221 is to disconnect one of the line coils from the common connection tag and add another anchor tag for the disconnected lead out wire. This has now been done so now it's the scary part of the job in which the CRT has to come out. Removal and refitting the CRT in the Ekco T221 and T231 was discussed in the Practical Television magazine.Like many other round Mazda CRTs the glass is very thin, much thinner than the rectangular 14" CRTs made by Mullard and many other firms. It wasn't until 1956 when Mazda introduced a rectangular faceplate CRT, strange because the company along with rival Mullard offered a rectangular 17" CRT in 1953. Brimar beat both them to it by offering set manufactures the C17BM in 1952. The metal CRT securing bands around the periphery of the faceplate are stuck fast and considerable care will have to be exercised to remove them, I've squirted WD40 on to the rubber strips between the glass bowl and the metal clamp bands, just leave the fluid to it's job and soften the rubber, daren't use force on fragile CRTs.